Function message_item_events 

fn message_item_events(
    item_id: &str,
    response_id: &str,
    output_index: u32,
    deltas: &[&str],
) -> Vec<(&'static str, Value)>

The events that stream one assistant message: the item, its content part, one event per delta, and the finished item. output_index is the message’s place among the round’s output items, so it depends on how many items the round emitted before this one.